The present invention relates to a process for the selective catalytic hydrogenation of carbonyl groups in aromatic aldehydes of the formula: ##STR2## wherein X is a carboxyl, methyl or a halogen, to yield the corresponding alcohols and methyl compounds.
Selective catalytic hydrogenation of aromatic aldehydes to give benzyl alcohols or the toluene compound which is produced as a result of hydrogenation, is of great importance for the preparation of organic intermediates in the field of fine chemicals.
This enables a method of obtaining these classes of products which is technically simple to accomplish and which represents a powerful alternative synthetic pathway for the preparation of substituted benzyl alcohols, which are otherwise obtainable using organic preparative methods via the crossed Cannizarro reaction of an aromatic aldehyde with formaldehyde.
In "Katalytische Hydrierungen" by F. Zymalkowski (Ferdinand Enke Verlag, Stuttgart, 1965; pages 103-105), palladium on activated carbon (Pd/C) and nickel catalysts are mentioned as suitable hydrogenation catalysts. Copper chromite catalysts for the selective hydrogenation of 4-chlorobenzaldehyde to give 4-chloro-1-hydroxymethyl-benzene are known from GB 1,144,257. Noble metal/titanium dioxide catalysts (NM/TiO.sub.2) for the hydrogenation of aromatic aldehydes are described in "Forming of High Surface Area TiO.sub.2 to Catalysts Supports" by Bankmann et al. in Catalysis Today 14 (1992) 225-242.
The selective hydrogenation of aromatic aldehydes has attained outstanding significance in the area of purifying terephthalic acid and also p-hydroxymethylbenzoic acid.
Terephthalic acid is prepared by radical catalyzed liquid phase oxidation of p-xylene in acetic acid using Co/Mn/acetate catalysts and bromine activation (K. Weissermel, H. J. Arpe in "Industrieller Organische Chemie"; Verlag Chemie, Weinheim 1988, pages 415-424). Terephthalic acid prepared in this way contains 4-carboxybenzaldehyde as an impurity, and this interferes with the subsequent polymerisation process intended to give polyesters. Therefore in the purification stage of the process, the crude terephthalic acid is dissolved under pressure in water at 275.degree. to 300.degree. C. and, in the presence of Pd/C or Rh/C catalysts, hydrogenated to give p-hydroxymethylbenzoic acid and p-methylbenzoic acid, or decarbonylated to give benzoic acid.
The reaction products mentioned above can be removed from the crude terephthalic acid by fractional crystallisation due to their greater solubility in water as compared with 4-carboxybenzaldehyde and in particular terephthalic acid.
Terephthalic acid is obtained in a so-called fiber-pure grade with a purity of 99.99%, which is then reacted, in the form of dimethylterephthalate, with diol components (preferably ethylene glycol) to give polyethylene-terephthalate (PET). The main use for this polyester is in the fiber industry.
According to EP 0 265 137, 0.5% Pd/C (coconut carbon) catalysts are preferably used for hydrogenating 4-carboxybenzaldehyde during purification procedures which conform to the relevant TA Standard.
Other catalysts which are known for purification procedures which conform to the relevant TA Standard are bimetallic Pt-Rh/C or Pd-group VIII metal/C mixed catalysts from DE 27 09 525, Ni/kieselguhr catalysts from U.S. Defensive Publication No. 880 007 (Official Gazette 1970, Vol. 880, No. 4, page 1162) and Pd, Pt, Rh, Ni, Ru, Co or mixtures thereof on metal supports from the group Ti, Zr, W, Cr, Ni or alloys of these metals from U.S. Pat. No. 4,743,577.
One problem when using the NM/C (NM=noble metal) catalysts described is the mechanical abrasion of black carbon fixed-bed catalysts which produces "black dots" and means the purified terephthalic acid does not comply with the specifications for fiber quality.
Another technically relevant example of an industrial application of the selective hydrogenation of aromatic aldehydes is the catalytic purification of p-hydroxymethylbenzoic acid, an important monomer for the preparation of poly-paramethylenebenzoate.
p-hydroxymethylbenzoic acid is prepared by the electro-chemical reduction of terephthalic acid and contains traces of carboxybenzaldehyde. Purification requires highly selective hydrogenation of 4-carboxybenzaldehyde to give p-hydroxymethylbenzoic acid.
Hydrogenolysis to give p-methylbenzoic acid must not take place since this terminates chain-formation during the subsequent polymerisation process, as does 4-carboxybenzaldehyde.
Ni/kieselguhr is used at 50.degree. C. according to U.S. Pat. No. 4,812,594 or Pt/C at 75.degree. C. in an aqueous, basic solvent at a pH between 7 and 8 according to U.S. Pat. No. 4,721,808.
The disadvantage of the processes described in the patents is that high p-hydroxymethylbenzoic acid selectivity is only achieved at low temperatures. However, the solubility in water of the participating reaction partners is too low at these temperatures, so a basic solvent has to be used. This in turn has the disadvantage that additional basic and acidic additives complicate the process and make it more expensive to operate.
A further industrial application for the selective hydrogenation of aromatic aldehydes is known from JP 53 002 441. This involves the hydrogenation of p-toluylaldehyde to give p-hydroxymethyltoluene in the presence of p-xylene-methylether and methyl p-methylbenzoate on a PtO.sub.2 catalyst doped with Fe.
An object of the present invention is to provide a process for the selective catalytic hydrogenation of aromatic aldehydes which avoids the disadvantages mentioned for the known processes.
Another object of the invention is to enable improved purification of crude terephthalic acid and p-hydroxymethylbenzoic acid and to permit the preparation of a whole class of fine chemicals. Another object of the invention is the provision of a method for preparing benzyl ether.